Can he improve? Sure it's possible, but 4 years of college and 5 years in the NFL is a long time, he's running out of time to improve. He will never be a Brees or such, that would require a huge jump in skill, and if he has that capability, why hasn't he done it?
Still, he hasn't had the greatest coaching, McCarthy, et al may be able to tailor the offense to his strengths, something Garrett never seemed to do, and don't get me started on Wade Wilson!
In any event, we're stuck with Dak for the next 4 years or so, barring injury, so we just have to hope what improvement he can/does make will be enough...